1. Field of the Invention
The present invention relates to a motor driving device for driving a DC brushless motor as used in a drive apparatus for magnetic disks, optical disks, magneto-optical disks, and the like.
2. Description of the Prior Art
As one conventionally known type of motor driving device for driving a DC brushless motor, a motor driving device of a sensorless type is used that detects the rotation position of the rotor of a brushless motor by exploiting the back electromotive forces appearing individually in the three-phase coils provided in the brushless motor. Such a sensorless motor driving device converts the currents fed to those three-phase coils into a voltage signal, and, on the basis of this voltage signal, performs pulse-width modulation. The resulting pulse-width-modulated signal is used to control a driver that supplies the three-phase coils with currents. Here, if the driver is controlled by directly using that signal, it is not possible to detect the back electromotive forces appearing in the three-phase coils during time periods in which that signal are off. To avoid this, it is necessary to control the driver by using a signal obtained by smoothing the pulse-width-modulated signal with a filter or the like.
This inconvenience is overcome in the motor driving device proposed in Japanese Patent Application Laid-Open No. H8-223970. In the motor driving device proposed in this Japanese Patent Application Laid-Open No. H8-223970, the currents supplied to the three-phase coils are detected by using a detection resistor, and, on the basis of the voltage signal appearing across this detection resistor, pulse-width modulation is performed. Then, the resulting pulse-width-modulated signal is fed back to a logic circuit to achieve desired control.
Thus, in the motor driving device proposed in Japanese Patent Application Laid-Open No. H8-223970, control based on pulse-width modulation is achieved through feedback of the currents supplied to the three-phase coils. However, here, control based on pulse-width modulation is exercised directly, and therefore the currents supplied to the three-phase coils are incessantly turned on and off. Thus, to achieve control based on pulse-width modulation through feedback of those currents, it is inevitable to use a filter circuit to smooth the voltage signal obtained from those currents.
An object of the present invention is to provide a motor driving device that controls the rotation speed of a motor in accordance with a signal produced on the basis of the back electromotive forces appearing in motor coils,
To achieve the above object, according to the present invention, a motor driving device is provided with: a driver for rotating a brushless motor by feeding currents to the motor coils provided, one for each phase, in the motor; and a speed detector for detecting the rotation speed of the motor by exploiting the back electromotive forces appearing in the motor coils . Here, the rotation speed of the motor is controlled by feeding a voltage signal output from the speed detector back to the driver after modulating the voltage signal by pulse-width modulation.
In this motor driving device configured as described above, for example when the load imposed on the brushless motor is light and the torque required by the motor is low, it is possible to control the motor through feedback based on its rotation speed.